1. Field of the Invention
This invention relates to methods for forming etch masks on substrates which are too large to efficiently employ photolithography techniques. Such etch masks may be used to form such structures as micropoint cathode emitters for field emission flat panel video displays, spacers for liquid crystal displays, quantum dots, or other features which may be randomly distributed on a surface.
2. State of the Art
For considerably more than half a century, the cathode ray tube (CRT) has been the principal device for electronically displaying visual information. Although CRTs have been endowed during that period with remarkable display characteristics in the areas of color, brightness, contrast and resolution, they have remained relatively bulky and power hungry. The advent of portable computers has created intense demand for displays which are lightweight, compact, and power efficient. Although liquid crystal displays (LCD""s) are now used almost universally for laptop computers, contrast is poor in comparison to CRTs, only a limited range of viewing angles is possible, and battery life is still measured in hours rather than days. Power consumption for computers having a color LCD is even greater, and thus, operational times are shorter still, unless a heavier battery pack is incorporated into those machines. In addition, color screens tend to be far more costly than CRTs of equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by the industry. Flat panel displays utilizing such technology employ a matrix-addressable array of cold, pointed, field emission cathodes in combination with a luminescent phosphor screen.
Somewhat analogous to a cathode ray tube, individual field emission structures are sometimes referred to as vacuum microelectronic triodes. Each triode has the following elements: a cathode (emitter tip), a grid (also referred to as the gate), and an anode (typically, the phosphor-coated element to which emitted electrons are directed). The cathode and grid elements are generally located on a baseplate, while the anode elements are located on a transparent screen, or faceplate. The baseplate and faceplate are spaced apart from one another. As the space between the baseplate and faceplate must be evacuated, a hermetic seal joins the peripheral edges of the baseplate to those of the faceplate.
Although the phenomenon of field emission was discovered in the 1950""s, it has been within only the last ten years that extensive research and development have been directed at commercializing the technology. As of this date, low-power, high-resolution, high-contrast, monochrome flat panel displays with a diagonal measurement of about 15 centimeters have been manufactured using field emission cathode array technology. Although useful for such applications as viewfinder displays in video cameras, their small size makes them unsuited for use as computer display screens.
Several engineering obstacles must be overcome before large screen field emission video displays become commercially viable. One such problem relates to the formation of load-bearing spacers which are required to maintain physical separation of the baseplate and the phosphor coated faceplate in the presence of external atmospheric pressure. Another problem relates to masking the baseplate in order to form the emitter tips. When the baseplate is no larger than the semiconductor wafers typically used for integrated circuit manufacture, the process disclosed in U.S. Pat. No. 5,391,259 to David Cathey, et al. works splendidly, as the mask particles can be formed from photoresist resin using a conventional photolithography process. However, when the baseplate is larger than those semiconductor wafers, conventional photolithographic techniques utilized in the integrated circuit manufacturing industry are much more difficult to apply. This disclosure is directed toward the problem of forming emitter tips on a large area baseplate.
Erie Knappenberger of Micron Display Technology, Inc. has proposed a new method for forming a mask pattern on a field emission display baseplate using beads or particles as the masking medium. As etch masks for a random pattern of similarly sized dots formed by dispensing glass or plastic beads suspended in a solution on an etchable surface are known to suffer from the problem of aggregation (i.e., multiple beads aggregating together on the surface), a nebulizer or atomizer is used to generate an aerosol containing particles. A monodispersed aerosol may be produced by utilizing a nebulizer or atomizer which produces droplets which are less than twice the size of the beads or particles within the mixture that is to be atomized. Alternatively, the mixture may be diluted so that the probability of two particles or beads being included within a single droplet is small. The aerosol thus created is then applied to a substrate, producing a uniform mono-layer of particles having substantially no aggregation. The particles may be used as a micropoint mask pattern which, when subjected to an etch step, forms field emitter tips for a field emission display or other micro-type structures. An alternative method for minimizing aggregation is to use two types of particles, one of which functions as a masking particle, the other which functions as a spacer particle. Thus, even if aggregation of particles is intentionally generated, the spacer particles may be removed by various techniques such as a chemical dissolution or evaporation, thereby minimizing aggregation of the masking particles themselves.
Another masking technique taught by U.S. Pat. No. 5,676,853 to James J. Alwan, utilizes a mixture of mask particles and spacer particles. The spacer particles space the mask particles apart from one another, and the ratio of spacer particle size to mask particle size and the ratio of spacer particle quantity to mask particle quantity control the distance between mask particles and the uniformity of distribution of mask particles.
An additional masking technique taught by U.S. Pat. No. 5,510,156 to Yang Zhao utilizes latex spheres which are deposited in a mono-layer on a surface, shrunk to reduce their diameters, and subsequently covered with an aluminum layer. When the reduced-diameter spheres are dissolved, apertures are formed in the aluminum layer, and the apertures are subsequently utilized to etch an underlying layer.
Still another masking technique is taught by U.S. Pat. No. 5,399,238 to Nalin Kumar. This technique relies on physical vapor deposition to place randomly distributed metal nuclei on a surface. The nuclei form a discontinuous etch mask on the surface of a layer to be etched.
Even under the best of circumstances, the use of the foregoing masking techniques will produce totally random patterns.
A more regular mosaic pattern may be produced by the process disclosed in U.S. Pat. No. 4,407,695 to Harry W. Deckman. Using this process, a mono-layer film of spherical colloidal particles is deposited on a surface to be etched. A spinning step which applies centripetal force to the particles is employed to improve packing density. The packed mono-layer is then ion etched to produce tapered columnar features. The tapering of the features results from continuing degradation of the colloidal particles during the ion etch step.
A masking technique similar to that patented by Deckman is disclosed in U.S. Pat. Nos. 5,220,725; 5,245,248 and 5,660,570 to Chung Chan, et al. This technique is disclosed in the context of fabricating an interconnection device having atomically sharp projections which can function as field emitters at voltages compatible with conventional integrated circuit structures. The projections are formed by creating a mono-layer of latex microspheres on a surface to be etched by spraying or pouring a colloidal suspension of the microspheres on the surface and, then, subjecting the mono-layer covered surface to either a wet etch or a reactive-ion etch.
What is needed is a simplified process for forming more regular mask patterns having no masking defects caused by two or more masking particles being too close to one another. The desired process should be capable of producing mask patterns which suffer little or no degradation during plasma etches. In addition, the process should be capable of forming masks which are usable for both reactive-ion etches and wet etches.
The heretofore expressed needs are fulfilled by a new process for forming a mask pattern. Beads, each of which has a substantially unetchable core covered by a removable spacer coating are used to form a discontinuous, regular hexagonal mask pattern. Each of the beads is preferably both spherical and of a particular size, as is each of the cores. For a preferred embodiment of the process, a reactive-ion-etchable material layer (hereinafter xe2x80x9cthe target layerxe2x80x9d) is coated with a thin thermo-adhesive layer. A bead confinement wall, or frame, is then secured to the peripheral edges of the target layer using one of several available techniques. For example, the confinement wall may be bonded to the thermo-adhesive layer, or it may be secured to the target layer with spring clips. In the former case, the confinement wall may be heated so that when it is placed on the thermo-adhesive layer, it bonds thereto. Beads are then dispensed onto the thermo-adhesive layer, in a quantity at least sufficient to form a hexagonally-packed mono-layer on the adhesive layer within the boundaries of the confinement wall. The bead-covered substrate is then subjected to vibration of a frequency and amplitude that will cause a settling of the beads to their lowest energy level, a state where optimum packing is achieved with a hexagonal mono-layer bead pattern in contact with the thermo-adhesive layer.
Optimum hexagonal packing having been achieved, the resultant assembly is heated, causing the layer of beads directly in contact with the adhesive layer to adhere thereto. The beads which are not in contact with the adhesive layer do not adhere to it. The unadhered beads are then discarded. This is accomplished, most easily, by inverting the assembly. They may also be removed by washing them from the assembly, after which the assembly is dried.
Spacer shell material is then removed from each of the beads, leaving only the cores visible in a top plan view. At least two methods may be employed to remove the spacer shell material between the non-etchable bead cores. The bead-coated substrate may be subjected to a first reactive-ion etch which etches away all of the spacer material except that which is beneath the cores and which is in bonded contact with the adhesive layer overlaying the substrate. The first reactive-ion etch chemistry is preferably selected such that it selectively etches the spacer material, but does not significantly etch either the cores or the target layer. If the target layer is etched simultaneously with the spacer material, uneven etching of the target layer will occur, as the areas of the target layer between the beads will etch first. The regions of the target layer closest to the cores will be the last areas exposed to reactive ion bombardment. Alternatively, the spacer material on the beads may be sublimable at elevated temperatures. Thus, as the coating on the beads sublimates, each non-etchable bead core will settle until it is eventually in direct contact with the adhesive layer. The core-masked target layer is then subjected to a second reactive-ion etch, which etches the target layer and forms a column beneath each core. If the target layer is laminar and is etched clear through to an underlying layer, a circular island of target layer material remains beneath each core. The cores are then removed, as well as any remaining spacer material beneath them.
In the case where a laminar target layer is etched clear through to an underlying layer, the circular islands of target layer material that remain may be used as a secondary mask pattern during a wet isotropic etch of the underlying layer. Such a combination of a unidirectional reactive-ion etch using the bead cores as a primary mask and an omnidirectional wet etch using the islands formed by the plasma etch as a secondary mask may be used to form micropoint cathode emitter tips in an underlying conductive or semiconductive layer.